Jacob M. Jacob M. Hartman

Thermonuclear (Type I) X-Ray Bursts Observed by the Rossi X-Ray Timing Explorer

We have assembled a sample of 1187 thermonuclear (type I) X-ray bursts from observations of 48 accreting neutron stars by the Rossi X-ray Timing Explorer, spanning more than 10 years. The sample contains examples of two of the three theoretical ignition regimes (confirmed via comparisons with numerical models) and likely examples of the third. We present a detailed analysis of the variation of the burst profiles, energetics, recurrence times, presence of photospheric radius expansion, and presence of burst oscillations, as a function of accretion rate. We estimated the distance for 35 sources exhibiting radius-expansion bursts, and found that the peak flux of such bursts varies typically by 13%. We classified sources into two main groups based on the burst properties: (1) both long and short bursts (indicating mixed H/He accretion), and (2) consistently short bursts (primarily He accretion), and we calculated the mean burst rate as a function of accretion rate for the two groups. The decrease in burst rate observed at > 0.06dot MEdd (~2 × 10^37 ergs s^−1) is associated with a transition in the persistent spectral state and (as has been suggested previously) may be related to the increasing role of steady He burning. We found many examples of bursts with recurrence times <30 minutes, including burst triplets and even quadruplets. We describe the oscillation amplitudes for 13 of the 16 burst oscillation sources, as well as the stages and properties of the bursts in which the oscillations are detected. The burst properties are correlated with the burst oscillation frequency; sources spinning at <400 Hz generally have consistently short bursts, while the more rapidly spinning systems have both long and short bursts. This correlation suggests either that shear-mediated mixing dominates the burst properties, or alternatively that the nature of the mass donor (and hence the evolutionary history) has an influence on the long-term spin evolution.

A decade of timing an accretion-powered millisecond pulsar: the continuing spin down and orbital evolution of SAX J1808.4-3658

The Rossi X-ray Timing Explorer has observed five outbursts from the transient 2.5 ms accretion-powered pulsar SAX J1808.4-3658 during 1998-2008. We present a pulse timing study of the most recent outburst and compare it with the previous timing solutions. The spin frequency of the source continues to decrease at a rate of (-5.5 {+-} 1.2) x 10{sup -18} Hz s{sup -1}, which is consistent with the previously determined spin derivative. The spin down occurs mostly during quiescence, and is most likely due to the magnetic dipole torque from a B = 1.5 x 10{sup 8} G dipolar field at the neutron star surface. We also find that the 2 hr binary orbital period is increasing at a rate of (3.80 {+-} 0.06) x 10{sup -12} s s{sup -1}, also consistent with previous measurements. It remains uncertain whether this orbital change reflects secular evolution or short-term variability.

Discovery of the Accretion-powered Millisecond Pulsar SWIFT J1756.9–2508 with a Low-Mass Companion

We report on the discovery by the Swift Gamma-Ray Burst Explorer of the eighth known transient accretion-powered millisecond pulsar, SWIFT J1756.9-2508, as part of routine observations with the Swift Burst Alert Telescope hard X-ray transient monitor. The pulsar was subsequently observed by both the X-Ray Telescope on Swift and the Rossi X-Ray Timing Explorer Proportional Counter Array. It has a spin frequency of 182 Hz (5.5 ms) and an orbital period of 54.7 minutes. The minimum companion mass is between 0.0067 and 0.0086 M☉, depending on the mass of the neutron star, and the upper limit on the mass is 0.030 M☉ (95% confidence level). Such a low mass is inconsistent with brown dwarf models, and comparison with white dwarf models suggests that the companion is a He-dominated donor whose thermal cooling has been at least modestly slowed by irradiation from the accretion flux. No X-ray bursts, dips, eclipses, or quasi-periodic oscillations were detected. The current outburst lasted ≈13 days, and no earlier outbursts were found in archival data.

Accelerated orbital expansion and secular spin-down of the accreting millisecond pulsar SAX J1808.4-3658

The accreting millisecond pulsar SAX J1808.4-3658 has shown a peculiar orbital evolution in the past with an orbital expansion much faster than expected from standard binary evolutionary scenarios. Previous limits on the pulsar spin frequency derivative during transient accretion outbursts were smaller than predicted by standard magnetic accretion torque theory, while the spin evolution between outbursts was consistent with magnetic dipole spin-down. In this Letter, we present the results of a coherent timing analysis of the 2011 outburst observed by the Rossi X-Ray Timing Explorer and extend our previous long-term measurements of the orbital and spin evolution over a baseline of 13 years. We find that the expansion of the 2 hr orbit is accelerating at a rate of \ddot{P}_b\simeq 1.6\times 10^{-20}\,s\,s^{-2} and we interpret this as the effect of short-term angular momentum exchange between the mass donor and the orbit. The gravitational quadrupole coupling due to variations in the oblateness of the companion can be a viable mechanism for explaining the observations. No significant spin frequency derivatives are detected during the 2011 outburst (|\dot{\nu }|\lesssim 4\times 10^{-13}\,Hz\,s^{-1}) and the long-term spin-down remains stable over 13 years with \dot{\nu }\simeq -10^{-15}\,Hz\,s^{-1}.

DISCOVERY OF A 552 Hz BURST OSCILLATION IN THE LOW-MASS X-RAY BINARY EXO 0748–676

We report the detection of pulsations at 552 Hz in the rising phase of two type-I (thermonuclear) X-ray bursts observed from the accreting neutron star EXO 0748?676 in 2007 January and December, by the Rossi X-ray Timing Explorer. The fractional amplitude was 15% (rms). The dynamic power density spectrum for each burst revealed an increase in frequency of 1-2 Hz while the oscillation was present. The frequency drift, the high significance of the detections and the almost identical signal frequencies measured in two bursts separated by 11 months, confirms this signal as a burst oscillation similar to those found in 13 other sources to date. We thus conclude that the spin frequency in EXO 0748 ? 676 is within a few Hz of 552 Hz, rather than 45 Hz as was suggested from an earlier signal detection by Villarreal & Strohmayer. Consequently, Doppler broadening must significantly affect spectral features arising from the neutron star surface, so that the narrow absorption features previously reported from an XMM-Newton spectrum could not have arisen there. The origin of both the previously reported 45 Hz oscillation and the X-ray absorption lines is now uncertain.

ACCRETION TORQUES AND MOTION OF THE HOT SPOT ON THE ACCRETING MILLISECOND PULSAR XTE J1807-294

We present a coherent timing analysis of the 2003 outburst of the accreting millisecond pulsar (AMXP) XTE J1807-294. We find a 95% confidence interval for the pulse frequency derivative of (+0.7, + 4.7) × 10–14 Hz s–1 and (–0.6, + 3.8) × 10–14 Hz s–1 for the fundamental and second harmonics, respectively. The sinusoidal fractional amplitudes of the pulsations are the highest observed among AMXPs and can reach values of up to 27% (2.5-30 keV). The pulse arrival time residuals of the fundamental frequency follow a linear anti-correlation with the fractional amplitudes that suggests hot spot motion both in longitude and latitude over the surface of the neutron star. An anti-correlation between residuals and X-ray flux suggests an influence of the accretion rate on pulse phase and casts doubts on the interpretation of pulse frequency derivatives in terms of changes of spin rates and torques on the neutron star.

ERRATUM: “DISCOVERY OF THE ACCRETION-POWERED MILLISECOND PULSAR SWIFT J1756.9-2508 WITH A LOW-MASS COMPANION” (2007, ApJ, 668, L147)

Table 1 contained one error. The time of ascending node T asc was incorrect due to a numerical conversion error in the preparation of the original table text. The correct value is T asc = 54264.78707(6). The correct value was used in all of the analysis leading up to the paper. As T asc is a purely fiducial reference time; the scientific conclusions of the paper are unchanged. We thank A. Patruno for bringing this error to our attention.